How to Size a Solar Battery Bank

Sizing a solar battery bank comes down to five numbers: daily energy consumption, desired autonomy days, battery depth of discharge, system voltage and inverter efficiency. Get these right and your system runs reliably for years. Get them wrong and you either overspend on batteries you do not need or, worse, run out of power on the second cloudy day.

This guide walks through the complete sizing process from load audit to final battery count. It works for any system: off-grid cabins, RV setups, van conversions, or grid-tied backup. If you want a quick answer, skip straight to the solar battery bank sizing calculator. To nail down your daily energy figure first, the off-grid load calculator walks you through a proper appliance audit. If you want to understand the math behind the result, read on.

Step 1: Audit Your Daily Energy Usage

The first number you need is your actual daily watt-hour usage. Not how many you think you use: how many you actually use.

List every device you plan to run on battery power. For each device, multiply its wattage by the hours per day you expect to use it. Add up the totals. Our electrical load calculator answers a different question, a dwelling's demand-factored service load by the optional method that municipal worksheets print, and takes no hours of use.

Two critical adjustments to make after your initial tally:

Inverter losses. Do not add anything to your appliance figures here. Record what each device draws, and let the sizing formula in Step 4 take the inverter out once, in its efficiency term: the solar battery bank sizing calculator puts the inverter efficiency you enter in that term, and your own inverter's datasheet prints that figure. Adding a margin to the appliance figures as well counts the same loss twice. DC-native devices (12V fridges, USB chargers, 12V LED lights) skip the inverter altogether, and for a bank feeding only those the calculator takes 100% in that input.

Seasonal variation. Your summer usage and winter usage may differ dramatically. A cabin that uses fans and a fridge in summer might switch to space heating and longer lighting hours in winter. Size for the season with the highest consumption unless you are willing to reduce loads during peak demand months.

Step 2: Choose Your Battery Chemistry

Battery chemistry determines how much of your rated capacity you can actually use. This is the single biggest factor in bank size and cost.

LiFePO4 needs fewer rated amp-hours than lead-acid to deliver the same usable energy, because you can safely discharge it deeper and its 12V block is 12.8V nominal against 12V for lead-acid. For one day of the 1,780 Wh daily load above at 12V, the solar battery bank sizing calculator returns 174Ah of LiFePO4 at 80% DoD against 297Ah of lead-acid at 50% DoD, so the LiFePO4 bank is about 59% of the lead-acid one in amp-hours (174 / 297 = 0.586). Both figures are for a DC load with no inverter in the path, 100% in the calculator's inverter efficiency. The calculator does not take the discharge rate: pull the same energy out faster and the lead-acid bank has to be bigger, because lead-acid delivers fewer amp-hours as the current rises. Use the Wh-to-Ah calculator to convert between units for your system voltage. If the bank you are pricing is sealed lead-acid, check what it actually delivers at your evening load with the AGM runtime estimator, which corrects for discharge rate.

The LiFePO4 option is lighter and is rated for more cycles in the table above, and over its lifetime it can cost less despite the higher sticker price. The costs in the table are our own estimates as of early 2026, an assumption for illustration, not a measured figure. For a detailed chemistry comparison, see our guide on LiFePO4 vs lead-acid runtime.

Step 3: Set Your Autonomy Days

Autonomy days = how many days your battery bank can power your loads without any solar recharging. This is your buffer for cloudy weather, snow coverage on panels, or system maintenance.

A system that can recharge another way (the grid coming back, a campground hookup, a generator) can carry fewer days; critical loads with no other source call for more. More autonomy days means more batteries, which means more cost and more weight. Going from 2 to 4 days of autonomy doubles the bank's amp-hours (4 / 2 = 2), and the extra days only count during a cloudy stretch longer than 2 days. Rolls ties its 1-2 days to that trade between battery cost and a generator: a smaller bank plus a propane generator for extended bad weather.

Step 4: Calculate the Bank Size

With your daily consumption, DoD, and autonomy days in hand, the formula is straightforward:

Bank capacity (Ah) = (Daily Wh × Autonomy days) / (Nominal battery voltage × DoD × Inverter efficiency)

The efficiency term matters: leave out a 90% inverter and a bank sized for two days delivers 1.8 days (2 × 0.90).

Using our example: 1,780 Wh/day, 2 days autonomy, a 12V LiFePO4 bank at the 12.8V nominal the solar battery bank sizing calculator uses for it and 80% DoD, with the AC loads going through a 90%-efficient inverter (an assumption for illustration, not a measured figure):

(1,780 × 2) / (12.8 × 0.80 × 0.90) = 3,560 / 9.216 = 386 Ah

Round up to the nearest available battery size. With 100Ah 12V LiFePO4 batteries at the same inputs, the solar battery count calculator returns 4 batteries, 1 in series per string × 4 parallel strings: 400Ah, a small margin above the 386Ah minimum.

The same calculation for lead-acid at 50% DoD:

(1,780 × 2) / (12 × 0.50 × 0.90) = 3,560 / 5.4 = 659 Ah

For 100Ah 12V lead-acid batteries at 50% DoD, the calculator returns 7 batteries, 1 in series per string × 7 parallel strings: 700Ah. On weight, the prints we hold are for different batteries: Battle Born prints 31 lbs for its 100Ah 12V LiFePO4 battery, so four of them weigh 124 lbs (4 × 31), and Renogy prints 512 lbs for four of its 12V 200Ah AGM or Gel batteries against "just under 190 lbs" for four of its 12V 200Ah LiFePO4 batteries. The solar battery count calculator takes your daily watt-hours, autonomy days, chemistry, system voltage, depth of discharge, inverter efficiency and one battery's capacity and voltage, and returns the number of batteries with the bank's total amp-hours and kilowatt-hours.

System voltage considerations

Higher system voltage means lower current for the same power, which allows thinner cables and reduces voltage drop over long cable runs.

• 12V: Simple, and it runs 12V appliances directly. It draws the highest current of the three for the same power, so a high-power inverter needs thick cables.
• 24V: Half the current of 12V for the same power (12 / 24 = 0.5), so thinner cables. Check that your charge controller and inverter take 24V.
• 48V: A quarter of the current of 12V for the same power (12 / 48 = 0.25), the lowest of the three, but fewer appliances run on 48V directly.

To build a 24V bank from 12V batteries, wire two in series. For 48V, wire four in series. You can then parallel these series strings to increase capacity. Never mix batteries of different ages, brands, or capacities in the same bank. Mismatched batteries create imbalance issues that reduce performance and lifespan of the entire bank.

Example: Sizing a Battery Bank for an Off-Grid Cabin

Sarah has a 3-season cabin in Colorado with no grid connection. Her daily loads total 2,400 Wh (fridge, lights, laptop, water pump, occasional microwave). She wants 3 days of autonomy because the cabin gets periodic multi-day cloud cover, and she has a small propane generator for extended bad weather.

She chooses LiFePO4 batteries at 80% DoD and a 24V system (to reduce cable costs for her 2,000W inverter). Her AC loads go through that inverter, taken at the same 90% as in Step 4, and the solar battery bank sizing calculator uses 25.6V nominal for a 24V LiFePO4 bank.

Bank capacity: (2,400 × 3) / (25.6 × 0.80 × 0.90) = 7,200 / 18.432 = 391 Ah at 24V

With 200Ah 12V LiFePO4 batteries at those inputs, the solar battery count calculator returns 4 batteries, 2 in series per string × 2 parallel strings (2S2P): 400Ah at 24V. She buys four and wires them that way. Renogy prints "just under 190 lbs" for four of its 12V 200Ah LiFePO4 batteries.

For solar panel sizing, Sarah needs enough panel capacity to recharge the bank on an average day. For Denver, NASA's POWER project prints an annual average of 5.6369 kWh per square metre a day on a south-facing surface tilted at the latitude. The example takes that figure for her cabin, with her panels at that tilt (an assumption for illustration, not a measured figure), and enters it as 5.64 peak sun hours; the calculator's help text asks for your worst month instead, and the winter section below covers that gap. Her 2,400 Wh a day goes through the same 90% inverter, so the array has to supply 2,400 / 0.90 = 2,667 Wh a day. With 200W panels, LiFePO4 at 80% DoD, a 24V bank, 3 days of autonomy and the page's default 14% system loss, the solar panel and battery sizing calculator returns 3 panels and the same 391Ah bank. The 14% is the system loss PVWatts prints as its default; the calculator also multiplies by a 98% charge controller efficiency, Victron's printed maximum for its SmartSolar MPPT 100/30 and 100/50, and by the cell-temperature factor 302 / 320 = 0.94375. Three 200W panels make 600 × 5.64 × 0.86 × 0.98 × 0.94375 = 2,692 Wh a day, 1% above the 2,667 Wh need, and she installs three. The calculator takes your daily energy usage, peak sun hours, system losses, panel wattage, battery chemistry, system voltage, depth of discharge, autonomy days and inverter efficiency, and returns the number of panels, the battery bank in amp-hours and the bank's energy in kilowatt-hours.

Common Battery Bank Sizing Mistakes

Sizing for rated capacity instead of usable capacity. A 200Ah lead-acid battery gives you 100Ah of usable energy at 50% DoD. If you sized for 200Ah, you will either damage the battery by over-discharging or run out of power at the 50% mark. Always calculate with usable capacity.

Ignoring inverter efficiency. An inverter loses part of what passes through it: through an inverter at the example's 90%, a 1,000W AC load draws 1,111W from the battery (1,000 / 0.90). That loss belongs in the sizing formula's efficiency term (0.90). Victron's RV solar sizing article instead tells readers to "add about 10-15% extra for any 230V appliances"; doing that on top of the efficiency term counts the same loss twice. Leave the term out instead and, as Step 4 shows, a bank sized for two days delivers 1.8 days at 90%.

Undersizing for winter. Solar production falls in winter: for a south-facing array at a 39° tilt in Kansas City, Missouri, NLR's PVWatts prints December's AC output at 72% of July's, on December solar radiation at 63% of July's, as our solar panel output guide explains in detail. If your panels barely recharge the bank in summer, you will have a deficit in winter. Either oversize panels for winter production, reduce winter loads, or plan to run a generator during the shortest days.

Mixing old and new batteries. Adding new batteries to an aging bank does not add the full rated capacity. The old batteries limit the performance of the new ones. Charge current flows preferentially to the lowest-resistance cells, which means new batteries get overcharged while old batteries remain undercharged. Build a new bank and retire the old one entirely.

Forgetting about temperature. Lead-acid batteries deliver less capacity in the cold. Trojan's rule of thumb is that "for every 15ºF (9ºC) below 80ºF (27ºC), capacity is reduced by 10%", and it calls the relationship basically linear: by that rule a 100Ah lead-acid battery at 32°F has 100 × (1 - 0.10 × (80 - 32) / 15) = 68Ah. A LiFePO4 battery's management system may block charging in the cold to protect the cells (Renogy sells LiFePO4 batteries with a "Low-Temperature Cut-Off"), and then you cannot recharge from solar on a freezing morning until the battery warms up.

Example: Weekend RV Boondocking Setup

Marcus takes his RV to desert campsites most weekends from October through April. His loads: 12V fridge (45W average), LED lights (30W for 5 hours), phone and tablet charging (15W for 3 hours), water pump (60W for 15 minutes/day), and a 12V fan (25W for 8 hours in warmer months).

Daily consumption: 1,080 + 150 + 45 + 15 + 200 = 1,490 Wh. He rounds up to 1,500 Wh to account for small miscellaneous loads.

Marcus wants 2 days of autonomy (arrive Friday evening, depart Sunday afternoon, about 48 hours) with some buffer. He chooses 12V LiFePO4 at 80% DoD because his RV is already wired for 12V. His loads all run on 12V DC with no inverter in the path, so the efficiency term is 1.00 (100% in the calculator).

Bank capacity: (1,500 × 2) / (12.8 × 0.80 × 1.00) = 3,000 / 10.24 = 293 Ah

With 200Ah 12V LiFePO4 batteries at those inputs, the solar battery count calculator returns 2 batteries, 1 in series per string × 2 parallel strings: 400Ah. He installs two in parallel, with headroom above the 293Ah minimum. A single 200W solar panel on the roof provides supplemental charging during the day, extending his autonomy beyond the calculated 2 days.

For weekend use, Marcus could have chosen lead-acid to save money upfront. But his batteries cycle about 30 weekends a year. At 30 cycles a year, the 600 cycles Victron prints for AGM in the table above last 20 years (600 / 30), and the 2,500 cycles it prints for LiFePO4 at 80% DoD last more than 80 years (2,500 / 30). The more often a bank cycles, the more the longer cycle life counts in the lifetime cost.

Check your own RV battery needs with the RV battery runtime calculator or calculate how many batteries your specific setup requires with the camper battery calculator.

Monitoring and Maintaining Your Battery Bank

A properly sized battery bank still needs monitoring. Without visibility into state of charge, voltage, and current flow, you are flying blind, guessing how much capacity remains instead of knowing. A battery monitor shows those readings: state of charge, voltage and current.

Battery monitors and shunt-based metering

A shunt-based battery monitor (like the Victron BMV-712 or Renogy 500A) installs on the negative battery cable and measures every amp flowing in and out of the bank. It tracks state of charge as a percentage, displays instantaneous power draw, records historical data like deepest discharge and cumulative energy throughput, and calculates estimated time remaining at the current load.

These monitors cost about $80-150 (an example price as of early 2026, an assumption for illustration, not a measured figure) and provide information that prevents both over-discharge (which damages batteries) and unnecessary generator starts (which waste fuel).

Voltage alone is an unreliable state-of-charge indicator, and for LiFePO4 it is not an indicator at all. Victron's Lithium Smart manual states that "state of charge monitoring is not built into the battery" and points the owner to a Lynx Smart BMS, a BMV or a SmartShunt; the makers that do publish resting-voltage charts do not agree with each other on the same reading. Lead-acid voltage varies more with SOC but is heavily affected by load: the same battery reads lower under load than after it has rested. A coulomb-counting shunt monitor avoids this problem entirely by tracking actual energy flow.

Cell balancing

Batteries wired in parallel should share current equally, but slight differences in internal resistance between batteries cause imbalances. Over time, one battery may charge fully while another lags behind, reducing overall bank capacity. For flooded lead-acid banks, equalization charging, a controlled overcharge, rebalances the cells, and Trojan's maintenance guide prints two instructions for it that differ: its Equalizing section, headed for flooded batteries, says "Trojan only recommends equalizing when low or wide ranging specific gravity (>0.030) are detected after fully charging a battery", while its tips for solar applications note that many charge controllers have equalization settings, say to "Equalize your batteries at least once per month for 2 to 4 hours, longer if your batteries have been consistently undercharged", and set the equalize charge at 16.2V for a 12V system in the table that follows.

LiFePO4 batteries handle balancing differently. Each battery has a BMS that balances its internal cells during the top-of-charge phase. For parallel LiFePO4 banks, make sure all batteries reach full charge regularly. A battery that never reaches full charge never triggers its BMS balancing cycle, and cell drift accumulates over time.

If your solar system consistently falls short of fully charging the bank (in winter or with undersized panels), run a generator or shore power connection periodically to bring all batteries to 100%.

Scheduled maintenance checks

Flooded lead-acid batteries lose water when they are charged and need topping up with distilled water: Trojan says to "Check your batteries once a month after installation to determine the proper watering schedule" and to add water after fully charging the battery. AGM and LiFePO4 batteries are sealed and need no fluid maintenance, but all battery banks benefit from regular connection inspections.

Check terminal torque, look for corrosion on copper lugs (white or green deposits), and verify that cable insulation shows no signs of heat damage or chafe. Resistance at a corroded terminal wastes energy and generates heat that can cascade into more serious problems. A short inspection on a regular schedule catches these before they turn into failures that are slow to diagnose.